A cascade impactor is an aerosol-sampling device that samples and separates the aerosol according to the aerodynamic properties of the particles. The typical cascade impactor has multiple collection stages arranged in series, with each collection stage having an orifice plate and a separate, removable collection surface positioned below the orifices (FIG. 1). With each successive stage, the total cross-sectional area of all of the orifices decreases in order to increase the velocity of the particle-laden air and thus the inertia of the entrained particles. The collection stages, or plates, serve as an impaction surface for the collection of the particles possessing inertia too great to be carried around the collection surface and onward to the next stage. Thus, successively smaller particles are collected on successive stages. The sizes of the particles collected on each stage, also referred to as the cut size, are primarily determined by the dimensions of the orifices above the stage and the volumetric flow rate through the impactor. Particles larger than the cut size are collected by inertial forces on the collection surface, while smaller ones remain entrained in the air stream to be collected on subsequent stages or a final collection filter. The cut size for a stage is the point of 50% collection efficiency.
Cascade impactors have been used in the pharmaceutical industry for many years (Hickey, A J. “Methods of Aerosol Particle Size Characterization” Chapter 8 in Hickey, A J ed. Pharmaceutical Inhalation Aerosol Technology, Marcel Dekker, NY 1992 pp. 219-253). The theory of operation is also known in the art (Marple, Va. and Rubow, K L “Theory and Guidelines” Chapter 4 in Lodge, J P and Chan, T L eds. Cascade Impactor, Amer Indust Hygiene Assoc 1986 pp. 79-101). The pharmaceutical industry has long adapted cascade impactors designed for use in industrial and environmental sampling to characterize inhalers and devices for respiratory drug delivery (USP <601> Aerosols). These accepted pharmaceutical sampling devices are large in size and are typically operated at flow rates ranging from 28 to 100 liters per minute. At these high flow rates, numerous orifices must be used on each stage, resulting in operating conditions outside of the ideal range, and consequently non-ideal calibrations. Further, these devices have large collection surfaces requiring the collection of relatively large amounts of material to satisfy analytical method requirements. Their size also creates inaccuracies in measurement due to interstage losses of material unaccounted for in routine analysis (Marple Va., Willeke K. Inertial impactors: theory, design, and use. In: Fine Particles, Aerosol Generation, Measurement, Sampling, and Analysis. Liu B Y H ed. Academic Press, NY, 1975). Further, these devices are not well suited for sampling aerosols intended for delivery to infants and children. Typical inhalation flow rates for children younger than 15 years of age range from 2 to 4 liters per minute (Coates, A. L., Tipples, G., Leung, K., Gray, M., Louca, E.; How Many Infective Viral Particles are Necessary for Successful Mass Measles Immunization by Aerosol; Vaccine; 24 (2006) 1578-1585). Infants, in particular, cannot be instructed to inhale at a rapid rate from an inhaler, and so must inhale at their normal tidal rate. Characterization of the dose and particle-size distribution from inhalation delivery devices operating at these low flow rates requires samplers, and in particular cascade impactors, operating at comparable flows.
Coating of Collection Surfaces with Adhesive Substances
To sample particles other than liquid droplets with a cascade impactor, it is generally accepted that a coating material must be placed on the collection stage. If not, the particles can bounce or be blown off or be re-entrained by the airflow, thus rendering the analysis of particle size erroneous. Typically the collection stages are coated by some means prior to sampling with a greasy, oily, or sticky substance, or a filter paper. Typically, the entire collection plate is coated by either dipping or spray or eyedropper application, and this can result in additional error for both gravimetric and quantitative chemical analysis, due to extraneous materials on the plate. Provided herein are embodiments of apparatuses and methods to apply coating material to only the small region opposite the orifices where the particles will impact, thus greatly reducing the quantity of extraneous materials required.